Photoacoustic Imaging (PAI) is a non-invasive medical imaging technique capable of viewing anatomical structures inside the tissue. Photoacoustic imaging may deliver both a high spatial resolution and a high contrast in medical imaging applications. To the contrary, conventional pulse-echo ultrasound medical imaging does not deliver both a high spatial resolution and a high contrast due to the similar acoustic properties of different body tissue. Compared to X-ray, photoacoustic imaging is a safer technique because it is non-ionizing and does not affect the molecules in the body.
Photoacoustic imaging is a relatively new technology and has not found its way too much into the clinical arena for use on humans, like ultrasound imaging has been for the last 50 years. One of the primary reasons is that the hardware has been less than desirable. Photoacoustic imaging makes use of infrared-induced ultrasound for constructing images of a target object. In this imaging process, the object to be imaged is flashed with a short near-infrared pulse, for example, a pulse in the order of nanoseconds. The long wavelength of near infrared light allows light to penetrate deep into the tissue. As the light is absorbed by tissue chromophores such as hemoglobin in blood, the tissue heats up and expands through a process called rapid thermoelastic expansion. This instantaneous tissue expansion creates ultrasonic waves which can be received by an ultrasound detector array placed outside the body. The received acoustic signals can be interpreted using beamforming algorithms to generate 2D or 3D images of the target tissue. PAI take advantages of the high contrasts of optical imaging and the spatial resolution of pure ultrasound imaging.
Photoacoustic imaging makes use of the infrared absorption rate difference of different kinds of tissue to create high-contrast images. Thus, different tissue absorbs different amounts of the infrared radiation and transmits ultrasound signals with dissimilar magnitude, phase, and/or frequency. These ultrasound signals are received by a transducer, summed up, and analyzed to produce images of the target tissue.
In photoacoustic imaging of humans and other animals with red blood cells, hemoglobin plays an important role in enhancing the image contrast because hemoglobin has a very high optical contrast when near infrared radiation is applied. As a result, high-contrast imaging of blood-containing structures in tissue, such as tumors or blood vessels, is one of the unique characteristics of photoacoustic imaging. By making use of this blood concentration/content related optical absorption, photoacoustic imaging may be useful for identifying diseases and/or abnormalities related to blood, including bleeding and (early-stage) cancer tumors. Thus, doctors can use photoacoustic imaging to recognize many problems that are difficult to identify via other techniques, such as, for example, ultrasonic imaging. In addition to viewing anatomical structure, photoacoustic imaging is capable of detecting composition of tissue and functional activities of an organ based on blood related infrared absorption rate differences.
In summary, traditional ultrasound imaging has low contrast in soft tissue, because the acoustic properties of different soft tissues are very similar. Photoacoustic imaging, on the other hand, delivers a much better contrast than ultrasonic imaging. Photoacoustic imaging is based on OPTICAL absorption for contrast, which depends primarily on the absorption spectrum of the tissue on the near-infrared light. The information is carried in the ultrasonic waves which allows for 3D imaging. One-way propagation of ultrasound is used to carry the information back to the ultrasound receiver. For photoacoustc imaging of live human or animal's tissue with red blood cells, hemoglobin provides significant help in boosting the contrast ratio. Due to the difference of imaging mechanisms, photoacoustic imaging delivers significant higher image contrast than traditional ultrasound imaging. The differences are especially significant on imaging of blood containing organs, e.g., cancer tumors.
Some conventional tubular ultrasonic internal imagers use piezoelectric transducers for transmitting and receiving ultrasound. Piezoelectric transducers used for medical imaging typically operate at a voltage higher than 100V. This high operation voltage requirement makes it difficult to fit these piezoelectric imagers into a miniature wireless unit for capsule endoscope applications; a wireless capsule endoscope powered by a battery is barely capable of providing such a high voltage even with the help of sophisticated voltage-pump circuits.
Recently, capacitive micromachined ultrasonic transducers (CMUTs) emerged as a promising alternative for piezoelectric transducers for medical imaging. In the ultrasound transmission process, the membrane of a CMUT is generally biased with a d.c. voltage. An a.c. signal is superimposed on this d.c. bias to cause a time-varying deformation of the membrane. This membrane deformation stirs the ambient media and transmits ultrasound. In a reception process, the membrane is also biased with a d.c. voltage, typically smaller in magnitude than that required for the transmission process. The impinging ultrasound deforms the membrane and causes a change in capacitance which is read out by the control electronics. Generally, the ultrasound transmission process consumes much more power than a reception process. For wireless endoscopic applications, lowering the power consumption such that the ultrasonic imager can operate a longer time with a battery may be desirable for the feasibility of the technology.
Currently, one conventional practice for photoacoustic imaging uses a near-infrared laser to illuminate a target biological object. The laser may comprise, for example, a Q-switched Nd:YAG laser or a laser diode coupled to an optical fiber or a lens. A separately located ultrasonic transducer array is used to receive the ultrasound emitted by the tissue. This arrangement may facilitate exploratory experiments, but may not be suitable for clinical uses, since the relative positions between the infrared beam, the target tissue, and the ultrasonic transducer array have to be manually adjusted. The manual alignment/adjustment is time consuming and generally does not deliver the accuracy needed for clinical use. Thus, these systems are not friendly to end users like medical doctors.
In addition to non-invasive detection, photoacoustic imaging has been used in invasive diagnosis for which part or the whole imager system is placed inside the tissue or an organ to be examined. Since it is close to the target being explored, an invasive photoacoustic imager is able to pick up the ultrasound signal before it attenuates appreciably through the tissue. It may therefore provide a better signal-to-noise ratio and deliver an image quality not available from a non-invasive modality.
One example of an invasive photoacoustic imager is an imager used for intravascular diagnosis. Several different modalities have been proposed for intravascular photoacoustic imaging. For example, a commercial side-looking intravascular ultrasound (IVUS) head was tested for photoacoustic imaging of a blood vessel by illuminating the blood vessel from outside of the patient's body using an infrared laser. While preliminary phantom images were obtained with this approach, this setup has at least two drawbacks. First, due to decay and scattering of infrared light in the tissue, laser illumination from outside of the patient's body is generally insufficient in strength for the photoacoustic imaging process on vessels deep inside the body. Second, the obstruction/shadowing of illumination by the IVUS head itself and/or an organ like a bone may introduce a dead viewing angle which could block a significant amount of view.
A possible solution to the aforementioned problem may be to integrate the light source with the ultrasound transducers, thereby providing the infrared illumination from the imager. Such a concept has been utilized in a variety of photoacoustic imaging designs. For example, a front-looking photoacoustic imaging probe that integrates an optical fiber with a polymer ultrasound transducer for intra-arterial imaging has been prototyped. The imaging probe was made up of a 600 μm core diameter optical fiber with a focused polymer transducer element constructed around the core of the fiber at its tip. This device was demonstrated to be capable of receiving the ultrasound signals transmitted by a human's finger tissue and nail in a photoacoustic process. However, due to being equipped with only one single element instead of an array of ultrasound transducers, no actual imaging was achieved.
Another possible solution included a customized optical fiber head for infrared excitation and ultrasound reception. On this all-optical photoacoustic probe, a thermal-sensitive Fabry-Perot polymer film sensor was mounted at the end of an optical fiber for receiving the ultrasound transmitted by a target object in a photoacoustic process. The optical fiber was used to introduce infrared radiation from an external laser source to the target object for stimulation as well as to guide the reflected infrared radiation (which carries the ultrasound information) to an external signal processing optoelectronic unit. This device was designed to look at the forward direction in which the optical fiber tip is pointing and was incapable of side viewing.
The photoacoustic imaging devices of the present disclosure solve one or more of the problems set forth above.